It actually came in before world war II and was one of the factors for Hitler's rise. Such as the Frankfurt School started in Germany in 1918 https://en.wikipedia.org/wiki/Frankfurt_School https://www.youtube.com/watch?v=4h_iC-UOCCc Of course oddly enough, despite what people say about Hitler, Fabian Socialist George Bernard Shaw was a fan, it wasn't that he disagreed with what Hitler did, but that he came to soon, he was a big fan of Stalin. Of course the Fabian society is the Labour Party in the UK, Fabian socialists are fans of Karl Marx who wrote the communist manifesto.

Communism started long before WW2. Support for communism in the US picked up around the time of WW1.

Honestly, I'm not sure what you are getting at. Communism existed for a long time before WWII. It gained major traction when the the Russians started a French-style revolution during WWI, which led to the formation of the USSR between the two wars. Also after WWI, the large empires that controlled central and eastern Europe broke up into several small independent countries. During WWII, Hitler took advantage of the several small (weak) countries, and set out conquering them all. At the end of the war, Stalin, who was basically less-racist yet somehow worse Hitler, filled the power vacuum in those little eastern European nations by installing puppet governments there. The people who lived there, after facing violent absolutist governments under imperialism, even more violent wars, even more violent Hitler, and then even worse more violent war driving Hitler back, were usually willing to settle for an oppressive communist regime, as long as it wasn't Stalin. Pieck, Rakosi, Hoxka, or friggin Ceausescu were okay, even though they were all followers of Stalin, and some of them were just as bad.

Communism as a national govt. started in Russia with the Russian revolution circa 1917. Russia was then named the Soviet Union and the U.S.S.R. Union of Socialist Soviet Republics. TBH unsure if its socialist soviet or soviet socialist.

The main reason Hitler opposed Stalin was that Stalin was trying to spread Communism into Europe and then the rest of the Western world. Stalin was a Bolshevik who rose to power during Germany's years of oppression under the Weimar Republic. During those years of the Weimar Republic, Germany was devastated by the reparations of the Versailles Treaty. Even though the Jews only made up roughly 1-2% of the population, they maintained a sizeable influence in business, culture, education, and other areas of society. Throughout this time it was feared that Germany would become a puppet state for either Britain or the Soviet Union. The reason Hitler came to power was that he represented the Germans' interest in returning Germany to a respectable state on the same level as Britain and other countries. This was something Britain nor the International Jews wanted to see. The moment Hitler passed the Enabling Act, the International Jews declared war on Germany. They continued to provoke Germany into a war against Stalin's Bolsheviks of the Soviet Union. The International Jews had no political state of their own, so they had no military to back up their threat of war against Germany. They got around this by manipulating Britain and the United States into going to war with Germany (just as they'd manipulated the United States to get involved in WW1). The mainstream narrative pushes the claim that Germany started WW2. No, they didn't. They were provoked into attacking Poland by Britain so that Britain could justify declaring war on Germany.